Pre-Design of a Solar Field with Parabolic Trough Collectors

The first estimation of the solar field is given by a simplified calculation of the performance of the thermosolar power plant, for what some information is needed:

* the location of the power plant,

* power block characteristics and

* the characteristics of the solar system (geometry, collector type, etc.).

The site where the solar thermal power plant will be built determines the geographical latitude and longitude, both necessary to know the incidence angle of the Sun, q>, on the aperture area of the solar field to design at noon and on the solstice. Using these variables in the power plant model

and assuming representative beam solar irradiance, Ed, and ambient temperature, Tamb, data at solstice noon at site, one can calculate the thermal power delivered by the solar field at that moment.

The power block determines the conditions (temperature and pressure) of the heat transfer fluid (HTF) at the solar field inlet and outlet as well as the required thermal power. The inlet temperature is set by the temperature of the condensed water from the Rankine cycle. The outlet temperature will be limited by the maximum bulk temperature of the solar field working fluid. On the other hand, the nominal electric power of the steam turbine determines the required thermal power, taking into account the efficiency of the whole thermal cycle, including not only the turbine but also the heat exchanger and the internal electric consumption for running the plant (around 10 %).

Solar field characteristics include the technical specifications of the chosen collector. These specifications include both dimensions — like its length and aperture width, the inner and outer diameter of the absorber tube and the roughness of its steel pipe — and thermal performance parameters — like peak optical efficiency, t]op0, incidence angle modifier, K(q), and heat losses, Qloss. A soiling factor, Fe, is also included in order to have an estimation of the influence of maintenance strategy on the solar field performance.

The orientation of the collector axis depends, mainly, on the pattern of plant operation and it can be limited by the size and geometrical shape of the plot available for the plant. The most used options are the North-South and the East-West orientation: while the North-South orientation produces a larger amount of energy annually, the East-West orientation gives less difference in energy output between summer and winter days.

The pre-design of the solar field consists of the calculation of the number of collectors per row and the number of rows needed to meet the nominal conditions required by the steam turbine, taking into account the information mentioned in the previous paragraphs. The number of collectors per row is a function of the temperature increase between the input and output of each row for what the temperature increase in every collector of a row has to be calculated assuming a nominal mass flow that guarantees a good heat transfer coefficient in the absorber tubes. The number of rows depends on the thermal power provided by every row and the total thermal power to be given to the Rankine cycle.